Epoxy resins are the most widely used adhesives. Nevertheless, the high dielectric constant (D_k) and dielectric loss (D_f) drastically limit their application in fifth-generation (5G) technologies. Herein, eugenol epoxy was grafted onto the vertexes of the octasilane POSS (POSS-H) via a hydrosilylation reaction, and the corresponding hollow glass microsphere (HGM) reinforced composites were prepared to simultaneously lower the D_k and D_f without sacrificing their mechanical properties. The obtained samples cured by methyl hexahydrophthalic anhydride exhibit low D_k (2.18) and D_f (0.016) at a frequency of 10 MHz due to the low polarity and large molecular volume of the POSS-H and HGMs. Moreover, the covalent bonding enables HGMs to be homogeneously distributed in the network without aggregation and phase separation. The samples exhibit excellent hardness (93.5 D), tensile strength (73.9 MPa) and compressive strength (95.1 MPa). This study has significant implications for the structural design of epoxy resins used in 5G technologies.